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A 1-mm$\phi$ sphere of yttrium iron garnet, a well-known ferro(ferri)magnetic insulator, contains $\sim 10^{19}$ net electron spins aligned in one direction. The spins, rigidly ordered by the exchange interaction and also interacting via the dipole forces, support collective excitations in the magnetostatic modes [1]. We control the quantum state of one of such modes coherently at the single magnon level by using a superconducting qubit. The qubit and the Kittel mode, the magnetostatic mode with spatially uniform spin precessions in the sphere, are strongly coupled via a microwave cavity mode, which results in the magnon-induced vacuum Rabi splitting of the qubit as well as Rabi oscillations between the qubit and the single-magnon excitation at resonance [2]. When the qubit and the Kittle mode are detuned, the dispersive interaction allows us to determine the magnon number distributions through the qubit spectroscopy [3]. These experiments demonstrate the potential of magnons as a quantum information carrier in the microwave domain. Coherent interaction of magnons with infrared light is also investigated [4,5].
[1] Y. Tabuchi et al., Phys. Rev. Lett. 113, 083603 (2014).
[2] Y. Tabuchi et al., Science 349, 405 (2015); C. R. Phys. 17, 729 (2016).
[3] D. Lachance-Quirion et al., arXiv:1610.00839.
[4] R. Hisatomi et al., Phys. Rev. B. 93, 174427 (2016).
[5] A. Osada et al., Phys. Rev. Lett. 116, 223601 (2016).
